The present invention pertains to capacitive pressure or force transducers, and in particular to the correlation of two signals with each other by means of such transducers.
It is useful to match one time-varying pressure signal with another. Pressure waves in fluids, such as audible sounds or SONAR responses, can be compared with previously recorded pressure wave signals in order to recognize or identify their signature. However, comparison of signal outputs from known sensors in real time, i.e. as the signal is produced, is not readily done except by converting the signals to digital representations and performing intense computational algorithms on the representative data. It is desirable to perform signal comparisons without a need for such computation.
Pressure or force transducers are known which rely upon a variation in capacitance between conductive plates which move with respect to each other under the influence of force or pressure. It is known to fabricate pressure transducers on substrates, similarly as an integrated circuit, in order to take advantage of batch processing capabilities, microminiaturization, and compatibility with integrated circuit manufacture. For example, U.S. Pat. No. 5,888,845 to Bashir, et al. describes a capacitive pressure transducer utilizing a membrane of heavily doped semiconductor crystal as a flexible, electrically conductive plate which shifts position with respect to a fixed perforated metallization layer. Pressure waves in such a device must be conducted through perforations in the metallization.
It is known to provide micromachined piezoelectric cantilevers to sense sound wave pressure. For example, U.S. Pat. No. 5,633,552 to Lee, et al. describes a pressure sensor micromachined from piezoelectric material for use as microphones and as microspeakers. However, piezoelectric material senses only change in position, and thus the output is related to the derivative of the position caused by the force, rather than being directly related to the position caused by the force. Moreover, the piezoelectric material is not compatible with some integrated circuit fabrication techniques.
The existing art provides transducers, but does not solve the problem of comparing signals using such transducers to reduce computation. Thus, a need exists for a method of comparing signals using a device which might function as a transducer.
The present invention addresses the above-noted needs and provides further benefits. The invention provides a method of comparing signals with each other by using physical movement of capacitive plates. The comparison output preferably represents a correlation function of the input signals. In one aspect, the invention compares a time-varying pressure with a second signal, the second signal representing, for example, a previously recorded time-varying pressure. In another aspect, the invention compares two signals to each other. In another aspect the invention provides devices which are useful to provide such comparison of signals by correlation function.
One embodiment of a sensor to practice the force comparison method of the present invention has a particularly useful relationship between applied pressure (force) and resulting capacitive plate spacing, such that an applied time-varying pressure can be correlated with an analog signal which may, for example, be derived from a reference time-varying pressure. In this embodiment, the distance d between two capacitive plates varies substantially as a linear function of an applied pressure p, i.e. xcex94d=k(p). An I(t) signal applied to the plates will therefore provide an output voltage VO(t) which is the correlation function of I(t) with p(t). Thus, if I(t) represents a reference pressure signal P0(t), which has perhaps been previously recorded, the presently applied pressure signal p(t) can be effectively correlated to P0(t). Also, since a time offset between I(t) and p(t) could cause a phase difference which would neutralize any correlation, a variable delay factor xcfx84 should be added to the signal I(t) so that it becomes I(t+xcfx84).
Any sensor constructed to yield a linear change in capacitive plate displacement as a function of pressure may be used with the inventive method of correlation taught herein. However, a preferred sensor is a microelectromechanical device constructed by semiconductor processing methods, in which regions of deposited metal provide the capacitive plates. Preferably, one of the metal regions is deposited upon a semiconductor substrate while the other metal region is supported by a beam, this plate-supporting beam being separated from the substrate by etching, and supported from the substrate on one or two ends. The plate-supporting beam is positioned under a diaphragm, which preferably contacts the plate-supporting beam through a contact point. In this embodiment, the area under the diaphragm is preferably evacuated, and then the diaphragm is sealed around the plate.
In one aspect, the invention permits real-time comparison of two electrical signals, which may accordingly represent any signal source. The distance d between two capacitive plates can be made to vary as a function of an applied voltage v(t) which corresponds to a first of the two signals. Then, a current corresponding to a second of the two signals may be applied to the capacitive plates, yielding a voltage which is indicative of the correlation function between the two signals . In this case, if V is the voltage applied to cause variation in the plate separation distance d, d=k1(V)⅔, and therefore the applied voltage must be preprocessed from the signal S(t) with which correlation is sought, such that the driving voltage V(t)=k2(S{fraction (3/2)}(t)).
A preferred embodiment of a device to practice this aspect of the present invention provides a micromechanical cantilevered beam supporting a conductive plate to which a voltage is applied with respect to an anchor plate. The voltage, which corresponds to the first signal, thereby applies a force to move the cantilevered beam in accordance with the first signal. The conductive plate is thus moved with respect to a capacitive sensing plate, so that a voltage, which is produced on the capacitive sensing plate by an applied current corresponding to the second signal, indicates a correlation of the first and second signals.
Thus, devices according to the present invention provide an ability to compare analog signals in real time, without digital conversions and intense computation, and indicate a correlation between the analog signals by a resultant voltage. In one aspect one of the signals is a physical force or pressure signal, but an immediate physical signal is not necessary to practice the comparison method of the invention.
Preferred embodiments of devices suitable for practicing the invention may be fabricated using batch integrated circuit manufacturing techniques. These embodiments perform the comparison function in a small region, permitting comparison of physically small force loci such as fluid pressure waves, including audible sounds, and also due to their small size permit a substantial frequency range of operation. Moreover, these embodiments lend themselves to formation of arrays of such sensors.